1. Field of the Invention
The present invention relates to a method of controlling a liquid ejecting apparatus, and a liquid ejecting apparatus.
2. Description of the Background Art
An inkjet printer which records an image on a recording medium such as printing paper by ejecting minute ink droplets from a plurality of nozzles of a head unit toward the recording medium while moving the recording medium relative to the head unit has been hitherto used (as disclosed, for example, in Japanese Patent Application Laid-Open No. 2013-71410). An inkjet printer for color printing includes a plurality of head units corresponding to inks of respective colors and arranged in a direction in which the recording medium is moved relative to the head units.
In the inkjet printer disclosed in Japanese Patent Application Laid-Open No. 2013-71410, each of the head units for the respective colors includes a plurality of heads arranged in a staggered configuration and each having a plurality of nozzles. In a liquid ejecting apparatus, such as the aforementioned inkjet printer, which ejects liquid toward a recording medium, ink to be ejected toward the recording medium is held in a meniscus shape in each of the nozzles provided in the heads. If the ink is not ejected for a long period of time, a volatile ingredient contained in the ink vaporizes at each nozzle, so that the ink is increased in viscosity or is solidified. As a result, there is apprehension about ejection failures of ink from the nozzles and about unevenness in density of an image recorded on the recording medium.
If ink containing a precipitable ingredient is not ejected for a long period of time, there is apprehension that the precipitable ingredient is precipitated inside the heads and in ink flow passages, so that the flow passages are narrowed down.
To solve the aforementioned problems, there has been hitherto known a maintenance technique (what is called a purge) in which a positive pressure or a negative pressure is applied to the ink inside the heads to force the ink out of the nozzles of the heads, thereby ensuring the stable operation of the heads.
Japanese Patent Application Laid-Open No. 2012-30516 discloses a “pressurized purge” technique in which a positive pressure is developed inside a head to force ink in the head out of nozzles (for example, paragraphs 0043 to 0052).
Japanese Patent Application Laid-Open No. 2012-30516 also discloses a technique in which a supply valve is interposed in a supply flow passage which connects the head and a sub-tank and in which the supply valve is closed, and is then opened after the pressure inside the sub-tank is increased to a designated pressure, whereby pressure waves provided to the head during the pressurized purge are made sharp.
Japanese Patent Application Laid-Open No. 2010-162783 discloses what is called a “negative pressure purge” technique in which caps are brought into contact with nozzle surfaces of heads and suction pumps connected to the caps are driven to decrease the pressure in spaces defined by the nozzle surfaces and the caps, thereby forcing ink out of the nozzles by suction.
As disclosed in paragraphs 0050 to 0052 in Japanese Patent Application Laid-Open No. 2010-162783, a positive pressure is developed inside the heads by applying pressure to the interior of the heads during the negative pressure purge. This allows the negative pressure purge, with the positive pressure held in the heads. It is said that this prevents air bubbles from being drawn into the nozzles when the caps are separated after the negative pressure purge. It is also said that the pressure applied to the heads is cut off by closing a supply path shut-off valve interposed in a supply flow passage connecting a sub-tank and the heads after the pressure application to the heads, to return the pressure in the heads to atmospheric pressure, whereby ink is prevented from dripping down after the separation of the caps.
In an inkjet printer having a plurality of heads, it is ideal to discharge ink from the heads at the same flow rate during the aforementioned “pressurized purge” because the problems of ejection failures of ink and unevenness in density are solved to the same degree. However, there are differences in the amount of ink discharge during the “pressurized purge” between the heads in actuality because of the differences in flow passage resistance from the sub-tank between the heads. This gives rise to problems of unevenness in density during image recording and excess ink consumption during the pressurized purge.
An inkjet printer having a plurality of heads presents another problem. When it is desired to eliminate an ejection failure in a specific head, the apparatus configuration as disclosed, for example, in Japanese Patent Application Laid-Open No. 2010-162783 is required to conduct a purge on all of the heads. To conduct the purge on the heads which require no purge because no ejection failure occurs therein results in the extra ink consumption.
When ink menisci are formed in nozzles by decreasing the pressure inside the heads from the sub-tank side after the pressurized purge, there are cases in which air bubbles enter the ink at the nozzles. This might cause an ejection failure in such nozzles.